Shrouds at the radial outer end of gas turbine blades are used for sealing the gap between the tip of the turbine blade and the turbine stator surrounding the turbine blade. By this measure a leakage flow through the gap between the tip and the stator is reduced. A typical shroud extends in the circumferential direction of the rotor and in the axial direction of the rotor along a substantial length of the turbine blade, in particular along its whole axial length, i.e. over a large area of the inner wall of the stator. In order to improve the sealing ability of the shroud there may be one or more sealing ribs, sometimes also called fins, which extend from a platform part of the shroud towards the inner wall of the stator.
As the shrouds, like the other parts of the turbine blades, are exposed to the hot pressurised combustion gas flowing through the flow path between the stator and the rotor one aims to sufficiently cool the shrouds to prolong their lifespan. A cooling arrangement in which air is blown out of bores in the stator towards the platform of the shroud for realising an impingement cooling of the shroud is described in US 2007/071593 A1.
EP 1 083 299 A2 describes a gas turbine with a stator and a rotor from which turbine blades extend towards the stator. At the radial outer tip of a turbine blade a shroud is located which faces a honeycomb seal structure at the inner wall of the stator. Cooling air is blown out of an opening in the stator wall into the gap between the shroud and the stator wall directly upstream from the honeycomb seal structure.